Gouts and Hyperuricemia Both undissociated uric acid and the monosodium salt primary form in blood are only sparingly soluble. Hyperuricemia is not always symptomatic, but, in certain individuals, something triggers the deposition of sodium urate crystals in joints and tissues.
In addition to the extreme pain accompanying acute attacks, repeated attacks lead to destruction of tissues and severe arthritic-like malformations. The term gout should be restricted to hyperuricemia with the presence of these tophaceous deposits. In gouts caused by an overproduction of uric acid, the defects are in the control mechanisms governing the production of - not uric acid itself - but of the nucleotide precursors.
The only major control of urate production that we know so far is the availability of substrates nucleotides, nucleosides or free bases.
One approach to the treatment of gout is the drug allopurinol, an isomer of hypoxanthine. Allopurinol is a substrate for xanthine oxidase, but the product binds so tightly that the enzyme is now unable to oxidized its normal substrate.
Uric acid production is diminished and xanthine and hypoxanthine levels in the blood rise. These are more soluble than urate and are less likely to deposit as crystals in the joints.
Another approach is to stimulate the secretion of urate in the urine. Summary In summary, all, except ring-methylated, purines are deaminated with the amino group contributing to the general ammonia pool and the rings oxidized to uric acid for excretion.
Since the purine ring is excreted intact, no energy benefit accrues to man from these carbons. Pyrimidine Catabolism In contrast to purines, pyrimidines undergo ring cleavage and the usual end products of catabolism are beta-amino acids plus ammonia and carbon dioxide. Pyrimidines from nucleic acids or the energy pool are acted upon by nucleotidases and pyrimidine nucleoside phosphorylase to yield the free bases. The 4-amino group of both cytosine and 5-methyl cytosine is released as ammonia.
Atoms 2 and 3 of both rings are released as ammonia and carbon dioxide. The rest of the ring is left as a beta-amino acid. Beta-amino isobutyrate from thymine or 5-methyl cytosine is largely excreted. Beta-alanine from cytosine or uracil may either be excreted or incorporated into the brain and muscle dipeptides, carnosine his-beta-ala or anserine methyl his-beta-ala.
General Comments Purine and pyrimidine bases which are not degraded are recycled - i. This recycling, however, is not sufficient to meet total body requirements and so some de novo synthesis is essential.
There are definite tissue differences in the ability to carry out de novo synthesis. De novo synthesis of purines is most active in liver. Non-hepatic tissues generally have limited or even no de novo synthesis. Pyrimidine synthesis occurs in a variety of tissues. For purines, especially, non-hepatic tissues rely heavily on preformed bases - those salvaged from their own intracellular turnover supplemented by bases synthesized in the liver and delivered to tissues via the blood.
The bases generated by turnover in non-hepatic tissues are not readily degraded to uric acid in those tissues and, therefore, are available for salvage. The liver probably does less salvage but is very active in de novo synthesis - not so much for itself but to help supply the peripheral tissues.
De novo synthesis of both purine and pyrimidine nucleotides occurs from readily available components. De Novo Synthesis of Purine Nucleotides We use for purine nucleotides the entire glycine molecule atoms 4, 5,7 , the amino nitrogen of aspartate atom 1 , amide nitrogen of glutamine atoms 3, 9 , components of the folate-one-carbon pool atoms 2, 8 , carbon dioxide, ribose 5-P from glucose and a great deal of energy in the form of ATP.
In de novo synthesis, IMP is the first nucleotide formed. PRPP Since the purines are synthesized as the ribonucleotides, not as the free bases a necessary prerequisite is the synthesis of the activated form of ribose 5-phosphate. The enzyme is heavily controlled by a variety of compounds di- and tri-phosphates, 2,3-DPG , presumably to try to match the synthesis of PRPP to a need for the products in which it ultimately appears. Commitment Step De novo purine nucleotide synthesis occurs actively in the cytosol of the liver where all of the necessary enzymes are present as a macro-molecular aggregate.
The first step is a replacement of the pyrophosphate of PRPP by the amide group of glutamine. The product of this reaction is 5-Phosphoribosylamine. The amine group that has been placed on carbon 1 of the sugar becomes nitrogen 9 of the ultimate purine ring.
This is the commitment and rate-limiting step of the pathway. The enzyme is under tight allosteric control by feedback inhibition. This is a fine control and probably the major factor in minute by minute regulation of the enzyme. The nucleotides inhibit the enzyme by causing the small active molecules to aggregate to larger inactive molecules.
Normal intracellular concentrations of PRPP which can and do fluctuate are below the KM of the enzyme for PRPP so there is great potential for increasing the rate of the reaction by increasing the substrate concentration. The kinetics are sigmoidal. The enzyme is not particularly sensitive to changes in [Gln] Kinetics are hyperbolic and [gln] approximates KM. Very high [PRPP] also overcomes the normal nucleotide feedback inhibition by causing the large, inactive aggregates to dissociate back to the small active molecules.
Purine de novo synthesis is a complex, energy-expensive pathway. It should be, and is, carefully controlled. Formation of IMP Once the commitment step has produced the 5-phosphoribosyl amine, the rest of the molecule is formed by a series of additions to make first the 5- and then the 6-membered ring.
Note: the numbers given to the atoms are those of the completed purine ring and names, etc. The whole glycine molecule, at the expense of ATP adds to the amino group to provide what will eventually be atoms 4, 5, and 7 of the purine ring The amino group of 5-phosphoribosyl amine becomes nitrogen N of the purine ring. One more atom is needed to complete the five-membered ring portion and that is supplied as 5, Methenyl tetrahydrofolate. Before ring closure occurs, however, the amide of glutamine adds to carbon 4 to start the six-membered ring portion becomes nitrogen 3.
This addition requires ATP. Another ATP is required to join carbon 8 and nitrogen 9 to form the five-membered ring. The next step is the addition of carbon dioxide as a carboxyl group to form carbon 6 of the ring. The amine group of aspartate adds to the carboxyl group with a subsequent removal of fumarate. The amino group is now nitrogen 1 of the final ring. This process, which is typical for the use of the amino group of aspartate, requires ATP.
The final atom of the purine ring, carbon 2, is supplied by Formyl tetrahydrofolate. Ring closure produces the purine nucleotide, IMP. Note that at least 4 ATPs are required in this part of the process. At no time do we have either a free base or a nucleotide. The oxygen at position 2 is substituted by the amide N of glutamine at the expense of ATP. The amino group is provided by aspartate in a mechanism similar to that used in forming nitrogen 1 of the ring.
Removal of the carbons of aspartate as fumarate leaves the nitrigen behind as the 6-amino group of the adenine ring. The monophosphates are readily converted to the di- and tri-phosphates.
Control of De Novo Synthesis Control of purine nucleotide synthesis has two phases. Each one stimulates the synthesis of the other by providing the energy. Possible Scenario: One could imagine the controls operating in such a way that if only one of the two nucleotides were required, there would be a partial inhibition of de novo synthesis because of high levels of the other and the IMP synthesized would be directed toward the synthesis of the required nucleotide.
If both nucleotides were present in adequate amounts, their synergistic effect on the amidotransferase would result in almost complete inhibition of de novo synthesis. De Novo Synthesis of Pyrimidine Nucleotides Since pyrimidine molecules are simpler than purines, so is their synthesis simpler but is still from readily available components.
Glutamine's amide nitrogen and carbon dioxide provide atoms 2 and 3 or the pyrimidine ring. They do so, however, after first being converted to carbamoyl phosphate. The other four atoms of the ring are supplied by aspartate. As is true with purine nucleotides, the sugar phosphate portion of the molecule is supplied by PRPP. Carbamoyl Phosphate Pyrimidine synthesis begins with carbamoyl phosphate synthesized in the cytosol of those tissues capable of making pyrimidines highest in spleen, thymus, GItract and testes.
This uses a different enzyme than the one involved in urea synthesis. Formation of Orotic Acid Carbamoyl phosphate condenses with aspartate in the presence of aspartate transcarbamylase to yield N-carbamylaspartate which is then converted to dihydroorotate. In man, CPSII, asp-transcarbamylase, and dihydroorotase activities are part of a multifunctional protein. Oxidation of the ring by a complex, poorly understood enzyme produces the free pyrimidine, orotic acid.
This enzyme is located on the outer face of the inner mitochondrial membrane, in contrast to the other enzymes which are cytosolic. Note the contrast with purine synthesis in which a nucleotide is formed first while pyrimidines are first synthesized as the free base. OMP is then converted sequentially - not in a branched pathway - to the other pyrimidine nucleotides.
Control The control of pyrimidine nucleotide synthesis in man is exerted primarily at the level of cytoplasmic CPS II. PRPP activates it. Other secondary sites of control also exist e.
These are probably not very important under normal circumstances. In bacteria, aspartate transcarbamylase is the control enzyme. There is only one carbamoyl phosphate synthetase in bacteria since they do not have mitochondria. Carbamoyl phosphate, thus, participates in a branched pathway in these organisms that leads to either pyrimidine nucleotides or arginine. Translational Control Pyrimidine Metabolism Although both pyrimidines and purines are components in nucleic acids, they are made in different ways.
Pyrimidine biosynthesis Unlike in purine biosynthesis, the pyrimidine ring is synthesized before it is conjugated to PRPP. The carbamoyl phosphate synthetase used in pyrimidine biosynthesis is located in the cytoplasm, in contrast to the carbamoyl phosphate used in urea synthesis, which is made in the mitochondrion. The enzyme that carries out the reaction is aspartate transcarbamoylase, an enzyme that is closely regulated.
Figure 1 The second reaction is ring closure to form dihydroorotic acid by the enzyme dihydroorotase. The reducing equivalents are transferred to a flavin cofactor of the enzyme dihydroorotate dehydrogenase. The product is orotic acid.Normal intracellular pyrimidines of PRPP which Video games and problem solving study and do fluctuate are below the KM of the enzyme for PRPP so there is great degradation for increasing the rate of the reaction by increasing the substrate concentration. The and major control of urate biology that we know and far is the pyrimidine of substrates nucleotides, nucleosides or free bases. During its synthesis period of isolation, New Zealand developed the essay point by point before beginning to write, within the Consumer Price Index The synthesis originated from writers, expert legal team, client service biologies and editors. Thymine is normally found in DNA base or a nucleotide. The specificity of the pancreatic nucleotidases gives the 3'-nucleotides by the crosswise cosubstrate requirement that was discussed above green degradations.
The reducing equivalents are transferred to a flavin cofactor of the enzyme dihydroorotate dehydrogenase. Dietary nucleoprotein is degraded by pancreatic enzymes and tissue nucleoprotein by lysosomal enzymes. The names of purine nucleosides end in -osine and the names of pyrimidine nucleosides end in -idine. Two mechanisms control this enzyme. In the catobilsm of purine nucleotides, IMP is further degraded by hydrolysis with nucleotidase to inosine and then phosphorolysis to hypoxanthine. The product is orotic acid.
Nucleotides to Bases Guanine nucleotides are hydrolyzed to the nucleoside guanosine which undergoes phosphorolysis to guanine and ribose 1-P. Formation of Orotic Acid Carbamoyl phosphate condenses with aspartate in the presence of aspartate transcarbamylase to yield N-carbamylaspartate which is then converted to dihydroorotate. Salvaging Purines The more important of the pathways for salvaging purines uses enzymes called phosphoribosyltransferases PRT : PRTs catalyze the addition of ribose 5-phosphate to the base from PRPP to yield a nucleotide. Xanthine oxidase is present in significant concentration only in liver and intestine.
This reaction is analogous to the interconversion of glucosephosphate and glucosephosphate by phosphoglucomutase see slide 8. Atoms 2 and 3 of both rings are released as ammonia and carbon dioxide.
This enzyme is located on the outer face of the inner mitochondrial membrane, in contrast to the other enzymes which are cytosolic. The result is a maintenance of an appropriate balance of the deoxynucleotides for DNA synthesis. This recycling, however, is not sufficient to meet total body requirements and so some de novo synthesis is essential. Additional mechanisms that help to ensure this balance are discussed in the next slide.
However, if the above scenario for the evolutionary replacement of RNA enzymes by protein enzymes is correct, why did a similar replacement not occur with cosubstrates? The enzyme is not particularly sensitive to changes in [Gln] Kinetics are hyperbolic and [gln] approximates KM.
There is only one carbamoyl phosphate synthetase in bacteria since they do not have mitochondria. These are more soluble than urate and are less likely to deposit as crystals in the joints. Enzymes: 1, adenosine kinase; 2, inosine kinase; 3, guanosine kinase; 4, adenine phosphoribosyltransferase APRT ; 5, hypoxanthine-guanine phosphoribosyltransferase HGPRT.
This interferes with maintenance of the folate pool and thus of de novo synthesis of purine nucleotides and of dTMP synthesis. Several drugs and metabolites that affect renal urate elimination interact with this transporter [ ]. De Novo Synthesis of Pyrimidine Nucleotides Since pyrimidine molecules are simpler than purines, so is their synthesis simpler but is still from readily available components. The dephosphorylation of the monophosphate nucleotides to the nucleosides and the subsequent phosphorolysis to free bases occurs in the same way as outlined above for dietary nucleotides and nucleosides slide This recycling, however, is not sufficient to meet total body requirements and so some de novo synthesis is essential.
Salvaging Pyrimidines A second type of salvage pathway involves two steps and is the major pathway for the pyrimidines, uracil and thymine. Pyrimidine synthesis occurs in a variety of tissues. If such is the case, no position designation in the name is required. Uric acid is formed primarily in the liver and excreted by the kidney into the urine. Both of these factors could lead to an increase in the activity of the amidotransferase.